The invention relates to a method for preventing a dangerous higher-frequency earth fault current for an electrical drive system which is operated in an electrical grid and has a power converter and an electrical drive machine, and to a fault current protective device for performing the method.
A failure or bridging of conductor-earth insulation due to earth faults in electrical devices, cables or other components of installations, which often also comprise electrical drive systems, can jeopardize the availability of the electrical drive system as a result of fire, for example. If the earth fault is caused by a person contacting an electrical conductor, dangerous earth fault currents can likewise occur which, in the form of unacceptably high contact currents, represent a life-threatening hazard to said person.
In order to protect against such hazards, in addition to solutions such as protective earthing or protective potential equalization, use is also made of fault current protective systems, often also referred to in general terms as residual-current circuit breakers (RCCBs). The purpose of these is to detect dangerous earth fault currents and, at the latest upon reaching limit values which are in part defined by standards or comparable regulatory specifications, to disconnect from the electrical grid that installation part which is jeopardized or hazardous to persons.
Although the cited residual-current circuit breakers are neither suitable nor intended to prevent an earth fault, they do in the event of an earth fault limit the height and the temporal duration of the developing dangerous earth fault current to a level which is not dangerous for persons and/or installations.
If a residual-current circuit breaker or a corresponding fault current protective device is designed for the fire protection of installations, for example, it must be assumed on the basis of fire protection knowledge that fire can break out in the event of an earth fault which has an electrical fault power of 60 W or more and is present over an extended period of time. For the purpose of fire protection in installations, a rated fault current of at most 300 mA is therefore usually stipulated as a limit value before tripping of the residual-current circuit breaker must occur. A rated fault current of 30 mA is usually specified as a limit value for the protection of persons. For specific applications, this limit value is also reduced to 10 mA.
If electrical drive systems comprising in particular frequency converters are used in installations, system-inherent properties of such drive systems often give rise to the difficulty that known residual-current circuit breakers cannot be used or can only be used with significant restrictions.
Such frequency converters often comprise power semiconductor switches which, depending on the operating mode, are switched by means of high switching frequencies greater than 1 kHz, wherein conductor earth capacitances of the electrical drive system, which are present due to the nature of the system, produce earth leakage currents having earth leakage frequencies of the same magnitude. However, the occurrence of these higher-frequency earth leakage currents often causes known residual-current circuit breakers to be tripped and consequently results in the electrical drive system being disconnected from the electrical grid, even though no earth fault is present in the electrical drive system. Such erroneous tripping jeopardizes the availability and hence the operational reliability likewise of the electrical drive system and/or the whole installation.
If the tripping of known residual-current circuit breakers is now restricted to detecting dangerous earth fault currents of low frequency up to e.g. 1 kHz, or if the sensitivity of detection in the higher frequency range is reduced, there remains a high hazard potential for persons and/or machine in respect of the fault current protection for installations comprising a corresponding electrical drive system, since dangerous earth fault currents of higher frequency cannot be detected or cannot be detected with the required degree of reliability.
The object of the invention is now to provide a method and a device for the fault current protection of an electrical drive system, which prevents the occurrence of dangerous higher-frequency earth fault currents more effectively than the known fault current protection and avoids erroneous tripping.